Liver cell transplantation has extensive potential for a variety of genetic and acquired conditions. Through pioneering work performed under this grant, we established for the first time that liver sinusoidal endothelial cells (LSEC) can be successfull transplanted. We developed suitable conditions to demonstrate engraftment of transplanted cells, including roles in this process of intrahepatic cell-cell interactions and cytokines/chemokines/receptors. Detailed analysis established that transplanted LSEC integrated in the endothelial lining of the liver and continued to express endothelial functions. Also, we established mechanisms regulating proliferation of transplanted LSEC in the liver. The ability of transplanted LSEC to synthesize and release FVIII permitted us to achieve permanent correction of hemophilia A in a mouse model. Subsequently, we established mechanisms in therapeutic gene transfer in transplanted LSEC that had been genetically-modified by lentiviral vectors. In other studies, we determined whether LSEC could originate from extrahepatic sources, e.g., bone marrow. These studies excluded that endothelial cells could originate from donor bone marrow-derived cells but we identified additional cell types, i.e., mononuclear cells, including Kupffer cells, as well as mesenchymal stromal cells derived from bone marrow with ability to express FVIII in lower levels. Finally, we identified antiviral mechanisms for hepatitisB and C replication, to consider whether disease-resistant hepatocytes and healthy endothelial cells could be simultaneously replaced. Therefore, insights in the biology of LSEC will offer suitable frameworks for cell/gene therapy in liver-related conditions. To advance cell/gene therapy, we now propose studies with human LSEC progenitor cells. Our hypothesis is that suitable endothelial progenitor cell populations can be isolated, expanded and manipulated in culture conditions from healthy donor tissues, followed by transplantation to replace liver sinusoidal endothelium, such that cell therapy in people will be advanced. As appropriate strategies are critical for inducing engraftment and proliferation in transplanted cells, we will develop studies in well-characterized and superb cell transplantation models. First, we will define properties of candidate human endothelial progenitor cells. This will be followed by examination of mechanisms to improve engraftment and proliferation in transplanted endothelial cells. These studies will permit examination of the cell therapy potential of human cells in hemophilia A mice. In this way, innovative approaches for cell therapy with human endothelial cells will overcome existing barriers in curing hemophilia and other disorders. Fundamental knowledge in respect to the properties and uses of human endothelial cells will advance insights in liver biology and liver cell therapy, which will be of enormous value for improving human health.